I agree with Jimmy Fixit'a answer. You should also consider using a pond coatng like Crommelin assuming that the tiles are solid color through and properly adhered. If you apply the Crommelin thick enough, you will create a new foundation for your final finish.

The previous answer is the approach you should be considering. Personally, after cleaning it, I would apply some sort of epoxy coating, or product that is a pond coating, I think Crommelin are the specialists for this, but Crommelin products may assume that the tiles are not fragile to begin with and also may not be clear, so may not be ideal, compared to something epoxy which if thick enough will create a whole new substrate to paint

The previous answer is the approach you should be considering. Personally, after cleaning it, I would apply some sort of epoxy coating, or product that is a pond coating, I think Crommelin are the specialists for this, but Crommelin products may assume that the tiles are not fragile to begin with and also may not be clear, so may not be ideal, compared to something epoxy which if thick enough will create a whole new substrate to paint

I agree with Jimmy Fixit'a answer. You should also consider using a pond coatng like Crommelin assuming that the tiles are solid color through and properly adhered. If you apply the Crommelin thick enough, you will create a new foundation for your final finish.